Method for the operation of an electron beam

ABSTRACT

A method is disclosed for the operation of a high-power electron beam for the vaporization of materials in a target. With this method, static and dynamic deflection errors are corrected. First, the static and dynamic deflection errors are ascertained by means of a teach-in process for concrete spatial coordinates and concrete frequencies of the deflection currents and stored in a memory. For the later operation, this stored data is used in such a way that input geometric data for the incidence points of the electron beam is automatically recalculated into corrected current values which bring about the exact incidence onto the input points. A corresponding procedure takes place with the input of frequencies for the deflection current. The input frequencies are automatically corrected in terms of frequency and amplitude in order to eliminate the frequency-dependent attenuation effects. Both in the correction of the static and in the correction of the dynamic deflection errors it is guaranteed by suitable interpolation methods that even the spatial coordinates and frequencies not considered in the teach-in process are taken into account. Finally, a method is specified with which it is possible by mere specification of a power distribution on a crucible surface to control the electron beam such that the specified data is satisfied.

INTRODUCTION AND BACKGROUND

The present invention pertains to a method for the operation of ahigh-power electron beam employed for the vaporization of materials.

Metals and metal alloys of high quality can be produced by means of anelectron beam melting process. The use of an electron beam as a heatsource for melting metals and alloys has the advantage that very complexmelting processes can be implemented, because the electron beam isdeflectable and thus can reach different places on the surface of ametal block or a metal melt.

Nearly any material can be effectively vaporized with the aid ofelectron beam technology. The vaporization rate is roughly 100 timesgreater than that of the sputtering process. Apart from the standardprocesses with aluminum, materials with a high melting point and highvaporization temperature are of particular interest for the electronbeam vaporization technique. Among these materials are, for instance,Cr, Co, Ni, Ta, W, alloys thereof or oxides like SiO₂, Al₂O₃, ZrO₂, MgO.Electron beam technology also provides the required stable and uniformvaporization rates for reactive vaporization, such as Al+O₂→Al₂O₃.

A particularly important field of application of electron beamvaporization is represented by the coating of large surfaces withvarious materials, for instance the coating of magnetic tapes with CoNialloys or the coating of films for the packaging of foodstuffs (SeeDE-OS 42 03 632 and the counterpart U.S. Pat. No. 5,302,208).

An additional field of application is the corrosion-preventive coatingof turbine blades, where, for instance, a layer 100 to 200 μm thick ofMCrAlY is applied and an additional heat-attenuating layer of 100 to 200μm of yttrium or stabilized ZrO₂ is added, so that the service of theturbine vanes is increased.

The main advantage of electron-beam coating lies in the high powerdensity in the focal point of the electron beam, which may amount to asmuch as 1 MW/cm². Due to this high power density, a high surfacetemperature results, so that even materials with a high melting pointcan be vaporized. Typically the focal point surface area is smaller than1 cm², so that only small vaporization zones are created. If thereforethe electron beam is stationary or the speed with which it scans thesurface to be vaporized is too low, the greater part of the electronbeam energy goes into the depths of the material, which does notcontribute to better vaporization.

The power distribution on the surface to be vaporized can be regulatedwith modern auxiliaries, whereby the layer thickness of thevapor-deposited material, for instance, can be optimized in a simplemanner by changing the pattern of the beam scanning.

Layers applied by electron-beam vaporization are often less dense thancomparable sputtered layers, and the properties of the layers can alsobe different. In order to improve the properties of the layers appliedby means of electron-beam vaporization, additional plasma support can beadded during the vapor-deposition process.

Due to the interaction of the electron beam with the residual gasparticles, the pressure in a coating chamber and the spacing between theelectron beam gun and the material to be vaporized, i.e., the beamlength, must not exceed a prescribed value. For acceleration potentialsof 20 to 50 kV, for instance, the pressure must not be greater than 10⁻²mbar. The length of the electron beam should not exceed 1 m. If higherpressures or longer electron beam lengths are required, the accelerationpotential should be increased.

A pressure increase at higher power levels can also be caused by theshield effect of material impurities, for instance, by H₂O or water ofcrystallization. Furthermore, some oxides break up in part into metaland oxygen. The pressure increase can change the layer properties ordefocus the electron beam. The vaporization materials should thereforebe optimized with regard to the shield effect of impurities and water.

Electron-beam guns with a power of up to 1000 kW and with accelerationpotentials of up to 160 kV are available. For coating purposes,electron-beam guns with powers of 150 to 300 kW and accelerationpotentials of 35 kV are generally employed. The electron-beam deflectionand focusing are generally carried out by means of magnetic coils. Boththe beam focusing and the beam deflection can be easily controlled byvarying the currents flowing in the magnetic coil.

In general, scanning frequencies of more than 10 kHz are used inelectron-beam welding. For coating applications, on the other hand, thecustomary frequency is around 100 to 1000 Hz, this frequency relating tothe fundamental frequency. If harmonics are present, frequencies of, forinstance, 10 kHz are included. Scanning frequency is understood to meanthe frequency at which an electron beam moves back and forth between,for instance, two points on the surface of a crucible.

In the controlling of a high-powered electron beam, essentially thefollowing aspects must be paid attention to: the power supply to thegun, the guidance of the electron beam inside the gun and guidance ofthe electron beam over the process surfaces.

Several methods of controlling a high-power electron beam are alreadyknown, in which there is provided a special deflection system (DE 42 08484 A1) with sensors for detecting the point of incidence of theelectron beam on a melt (EP 0 184 680, DE 39 02 274 C2, EP 0 368 037, DE35 38 857 A1). Also, deflection systems with more than one electron beam(U.S. 4,988,844) or electron-beam positioning regulators with magneticfield sensors (DE 35 32 888 C2) have been proposed.

Also known is a control of a high-power electron beam carried out bymeans of a microprocessor, in which conventional hardware is operated bysoftware that is designed for uniform beam dispersion and greatflexibility in the carrying out of melting instructions or formulas (M.Blum, A. Choudhury, F. Hugo, F. Knell, H. Scholz, M. Bähr: Applicationof a New Fast EB-Gun Control System for Complex Melting Processes, EBConference, Reno/USA, Oct. 11-13, 1995). The essential characteristicsof the high-frequency controlled electron-beam system are a thermalcamera and measuring unit for the element concentration in the gasphase. This control system can be applied in a variety of ways, forinstance, for hearth melting of titanium or in drop melting of tantalum.It is also suited for simultaneous control of several melting furnace,which can be equipped with up to 5 electron-beam guns. With it, it isalso possible to implement an electron-beam process with preciselydefined surface temperature distribution even for asymmetric meltingarrangements, for instance, in horizontal drop melting, where on oneside, the material to be melted is supplied via a water-cooled coppertrough, or in another electron-beam arrangement, where a high inputenergy results at one side due to the overflowing melt material. Thecontrol is also accomplished in this known arrangement by means of aconventional PC, which is operated by way of a software based onWINDOWS®.

In a refinement of the above-described control of a high-power electronbeam, an electron beam scanning and control system is used, with whichthe electron-beam scanning rate is directly controlled (M. Bähr, G.Hoffmann, R. Ludwig, G. Steiniger: New Scan and Control System(ESCOSYS™) for High-Power Electron Beam Techniques, Fifth InternationalConference on Plasma, Surface Engineering, Garmisch-Partenkirchen,September 1996). This control system, which relies on so-called“internal intelligence,” has two essential characteristics. Onecharacteristic pertains to error compensation. Here the behavior of theelectron beam is first “trained,” wherein one starts on a screen withlow power. After this “training process,” the frequency attenuation anddeflection errors of the electron-beam gun are automatically compensatedfor. A circular pattern of the beam remains a circle and not, say, anellipse in the crucible, even at different angles of incidence. The sizeof this circle remains constant even if the scanning frequency ischanged. The deflection error compensation is performed by applying a2×n-dimensional polynomial function. The frequency attenuation iscompensated with respect to amplitude and phase-angle rotation byapplication of the Fast Fourier Transform algorithm. Thus, not onlygeometric patterns, but even very precise patterns are compensated.Nonetheless, the system operates with a frequency limitation of 10 kHz,which permits cycle frequencies of up to 1 kHz. This minimizes thenecessity of a frequency-attenuation compensation. Alongside theaforementioned error compensation, the direct input of the powercompensation for a given surface is essential. With the known system,vapor-deposited layers of great uniformity can be achieved at a highspeed. For a reactive Al₂O₃ process, for instance, a coating speed of 10m/sec is possible. Additional details on how the aforementioneddeflection-error compensation and frequency-attenuation compensation areachieved were not given in the aforementioned presentation.

Starting from the above-described state of the art, it is an object ofthe present invention to make it possible to deflect the electron beamautomatically and with error-compensation for preset scanning or powerpatterns.

SUMMARY OF THE INVENTION

The above and other objects of the invention can be achieved by themethod for the operation of a high-power electron beam which is employedfor the vaporization of materials in a crucible or the like, wherein oneor more deflection units are provided for the electron beam and theelectron beam is directed at essentially constant intensity onto thematerial to be vaporized. The electron beam can be guided at aspecifiable velocity over various points of the surface of the materialto be vaporized, according to selected geometrical coordinates of thepoint on the surface of the material to be melted. The selectedgeometrical coordinates (x,y; r,φ) are transformed into correcteddeflection currents (I_(x),I_(y); I_(r),I_(φ)) and are supplied to thecorresponding deflection units.

It is a further feature of the invention that the geometrical powerdistribution of the electron beam on the surface of the material to bevaporized is established;

then the geometrical coordinates (x,y; r,φ) on the surface of thematerial to be vaporized corresponding to the power distribution areascertained, and the ascertained geometrical coordinates (x,y; r,φ) aretransformed into corrected deflection currents (I_(x),I_(y);I_(r),I_(φ)) and supplied to the corresponding deflection coils.

A still further feature of the present invention resides in the methodfor the operation of the high-power electron beam as described abovewhere the geometrical coordinates of the points on the surface of thematerial to be melted which the electron beam is to approach one afterthe other are selected, and the actual association of the geometricalcoordinates (x,y; r,φ) to deflection-current coordinates (I_(x),I_(y);I_(r),I_(φ)) is ascertained in the static operation of the electronbeam;

the actual association of the geometrical coordinates (x,y; r,φ) todeflection-current coordinates (I_(x),I_(y); I_(r),I_(φ)) is ascertainedin the dynamic operation of the electron beam;

correction parameters are ascertained which determine the deviationsbetween ideal current coordinates (I_(xi),I_(yi); I_(ri),I_(φi))associated with the geometrical coordinates (x,y; r,φ) and the actualcurrent coordinates (I_(x),I_(y); I_(r),I_(φ)); and

the ideal current coordinates are corrected in order to control theelectron beam with the aid of the ascertained correction parameters

Still further, in accordance with the present invention the correctedcurrents for a point (x′,y′) of the surface to be vaporized that werenot taken into account in the training process are determined from theequation $\begin{matrix}{{{current}\quad {amplitude}_{x^{\prime}y^{\prime}}} = \quad {\sum\limits_{i,{j = 0}}^{n}{a_{i,j} \cdot ( {{spatial}\quad {coordinate}\quad x} )^{i} \cdot}}} \\{\quad ( {{spatial}\quad {coordinate}\quad y} )^{i}}\end{matrix}$

(spatial coordinate x)^(i)·(spatial coordinate y)^(i)

wherein

α(I,ω=0) is the deflection angle for direct-current deflection of theelectron beam and

α(I,ω=1·ω_(T)) is the deflection angle for alternating-currentdeflection of the electron beam.

Yet another feature of the invention is that prescribed geometricalpoints on the surface of the material to be vaporized are associatedwith electric power of the electron beam striking it, wherein the poweris determined by the intensity and the velocity of the electron beam;and

the coordinates for an ideal geometrical motion pattern of the electronbeam are ascertained, which guarantees that the geometrical points aresupplied with the prescribed powers of the electron beam.

The advantage achieved with the invention is, in particular, that thegeometrical path of the electron beam or the power density produced byit on the predetermined melt surface is freely selectable anderror-compensated. The user need no longer take into account errorswhich may occur and can make the input directly in spatial coordinates.Additionally, the user can define the power distribution directly andneed no longer, as previously, ascertain the power distribution onlyexperimentally by a suitable combination of geometrical deflectionpatterns. It is, moreover, possible with the invention to use a closedcontrol loop. If, for instance, the vaporization rate is measured insitu in a vaporization process and a new power distribution is generatedfrom it by means of a control algorithm in order, for instance, toachieve a more uniform coating, then such a control loop can readjustvery precisely if an error correction was previously performed. If theseerrors are not corrected, residual errors remain even for a closedcontrol loop.

For melting, it is possible with the aid of the invention, for instance,for the temperature distribution on a crucible or ingot to be controlledor regulated if an appropriate measuring system is available. In thisway, material structures and undesired vaporization losses of alloycomponents can be better optimized.

The invention involves a passive system, that is to say, no measurementof the point of incidence of the electron beam on the material to bemelted is undertaken. Rather, the system knows the points of incidencethrough a recognition (“teach-in”) performed one time at the start ofthe melting process. The invention can, however, also be combined withautomatic measurement systems, which measure the points of incidence ofthe electron beam directly. In the implementation of the passive system,a deflection pattern is first defined in spatial coordinates andinterim-stored in a memory. The deflection speed of the electron beam iscalculated from the specified deflection frequency of the electron beamand the distances between the defined points on the surface of thematerial to be melted. A special algorithm, which, in particular,contains an error-correction algorithm, then transforms the deflectionpattern defined in spatial coordinates into such a deflection patternthat is defined in current values for the deflection coils. Here, too,the deflection speed results from the additionally specified frequency.The current pattern thus obtained can then be supplied directly to acurrent amplifier which drives the magnetic deflection coils and thusdeflects the electron beam magnetically. The frequency-dependentattenuations arising from eddy currents and the frequency-dependentnonlinear distortions in the current amplifier, as well as any othernonlinearity in the frequency response between pulse output and electronbeam is not taken into account in this regard.

BRIEF DESCRIPTION OF DRAWINGS

The present invention will be further understood from the followingdrawings, wherein:

FIG. 1 is a perspective view of a schematic representation of anelectron-beam melting or electron-beam vaporization device withdeflectable electron beam;

FIG. 2 is a perspective view of a melting crucible, over the surface ofwhich a deflectable electron beam is guided;

FIG. 3 is a perspective view of a screen plate that is arranged above acrucible for calibration purposes;

FIG. 4a and 4 b are graphical representations of the actual x,y currentcomponent in order to achieve a specified pattern;

FIG. 5 is a schematic representation for explanation of the growingdeflection error of an electron beam with increasing deflectionfrequency;

FIG. 6a and 6 b are representations of the frequency-attenuationcompensation, in which the current amplitudes are increased radially andtangentially with increasing frequency;

FIG. 6c and 6 d are the frequency spectrums of the x and y currentcomponents of a corrected deflection-current profile;

FIG. 7 is a schematic representation of an arrangement in which staticand dynamic correction of the deflection errors is undertaken;

FIG. 8 is a specified power distribution for a rectangular crucible;

FIG. 9 is the electron beam path for a power distribution according toFIG. 8;

FIG. 10 is a specified power distribution for a round crucible;

FIG. 11 is an electron beam path for a power distribution according toFIG. 10;

FIG. 12 is a schematic representation of the driving of twoelectron-beam guns according to the invention;

FIG. 13 is a schematic representation of a closed regulation system; and

FIG. 14 is a geometrical representation of the various process pathsaccording to the invention.

DETAILED DESCRIPTION OF INVENTION

The present invention will now be described in further detail.

In FIG. 1, the principle of the high-power electron-beam gun 1 isschematically represented. A heating filament that is connected to avoltage U_(f) and emits electrons 3 is labeled 2. Below the filament 2,a stationary and usually block-shaped cathode 4 is arranged. This isheated by an electron current to the point where this block 4 itselfemits electrons 6. A high voltage U_(b), by which the electrons 6 areaccelerated, is present between the block 4 and an anode 8. A Wehneltcylinder 5 influences the electrostatic field in such a way that theelectrons are focused into a hole of the anode 8 and thus a concentratedelectron beam 7 is generated. This electron beam is further concentratedby means of magnetic lenses 9,10. A magnetic deflection unit 11 that issupplied by a current I_(d) which comes from the power supply 12. Thispower supply 12 can supply direct or alternating current, the amplitudeand/or frequency of the current being variable.

Beneath the electron-beam gun 1, a crucible 13, in which metal or ametal alloy 14 is located, is shown in cross section. The deflectedelectron beam 7′, which has an angle of deflection a to the horizontalaxis 16 in FIG. 1, strikes the surface 15 of this metal 14 at a point21.

A thermal camera 20 makes it possible to detect the temperaturedistribution on the surface 15 of the metal 14.

Only the lower part 22 of the electron-beam gun 1 is shown in FIG. 2,wherein the deflection unit 11 is not separately illustrated; it islocated inside part 22. The crucible 13 and the metal surface 15 areshown in perspective.

One recognizes here that the electron beam 7′, shown here only as aline, can be deflected in such a manner that it passes over the entiresurface 15. Individual points of incidence of the electron beam 7′ onthe surface 15 are labeled P₁-P₈ .

The points of incidence P₁-P₈ can be defined on the surface 15 with theaid of x,y coordinates or polar coordinates. If the length of theelectron beam 7′ is included in the establishment of the pointcoordinates, then spherical coordinates are a logical choice. Here thedistance 23−P₁ establishes the distance of the electron beam from theexit from the electron-beam gun to the point P₁, while a first angle α,not shown, designates the deviation of the beam from the central axisand a second angle β, likewise not shown, designates the angle ofrotation in, say, the clockwise direction. As is known from thetechnology of television tubes, an electron beam can be deflected to acertain point if certain currents flow through the x,y deflection coils.There is thus an unambiguous association between the current through thex,y deflection coils and the point of incidence of the electron beam 7′.

This fixed association of spatial coordinates and ideal currentcoordinates applies however only to a stationary deflected electron beam7′, that is to say, only if the electron beam 7′ does not move and if nodisturbances appear. If the electron-beam gun is, for instance, set upat an incline to the crucible 13 or if additional interfering electricalor magnetic fields appear at the crucible 13, the electron beam 7′cannot reach the prescribed points P₁-P₈ even if its deflection unitsare supplied with the currents that are associated with these pointsunder ideal conditions. In vaporization processes, however, theelectron-beam gun is generally not positioned at right angles andcentered with respect to the crucible, in order to minimize internaldamage to the gun tube. Thereby, for instance, a square deflectionpattern on the surface 15 is distorted into a bent trapezoidaldeflection pattern, or a deflection pattern input as a circle becomes anellipse. In order to generate a square or a circle on the material to beevaporated, despite this, corrections must consequently be made to theideal current parameters, that is to say, the deflection units must notbe supplied with the same currents as in the case of an exactlysymmetrical orientation of the electron beam and purely inductivedeflection units.

In the vaporization processes described here, the intensity of theelectron beam 7′ is always constant or can be varied only slowly. Thepower input into the surface 15 by the beam 7′ is determined by thespeed with which it moves from one point to the next. One speaks, forinstance, of the dwell time of the electron beam on a point of thesurface, because the beam is not exactly a point and thus remains withits diameter on a point for a certain time.

In order to determine the aforementioned static deflection error and itsinfluence, the deviation of the actual deflection from an idealdeflection must be established. The ideal deflection can be easilydetermined by calculation, but not the actual deflection. This istherefore determined by means of a training process.

FIG. 3 shows a sketch to explain the training (teach-in) process for acorrection of deflection errors. Such deflection errors can, as alreadymentioned, be provoked by a slanted position of the electron gunrelative to the central crucible axis or by additional magnetic fieldson the crucible. The correction of such deflection errors is also calleda static correction because it also must be undertaken with a non-movingelectron beam. The electron-beam gun 22 and the electron beam 7′ areonly schematically shown in FIG. 3. Essential to the training orteach-in process is a marker sheet 50 that is arranged above a crucible,not shown and has the marker points P₉-P₁₉. These marker points aretargeted manually and individually by varying the deflection currentsfor the electron beam 7′.

If the electron beam 7′ assumes the position shown in FIG. 3, the knownspatial coordinates x₁₀,y₁₀ for P₁₀ are stored in a memory 51.Associated with these spatial coordinates are current coordinatesI_(x10),I_(y10), that is, the current for the x and y deflections whichcause the electron beam 7′ to assume the position shown in FIG. 3. Theseare the actual flowing currents, which differ from the ideal currentsassociated with an x,y coordinate and already contain an errorcorrection. The amplitudes of the actually flowing currentsI_(x10),I_(y10), which are necessary in order to reach a prescribedpoint, are likewise stored in the memory 51 and associated with thespatial coordinates of P₁₀. The same procedure is now followed for thespatial coordinates P₉ and P₁₁-P₁₉ so that ultimately every point P₉-P₁₉in the memory is associated with certain deflection currentsI_(x9),I_(y9) . . . I_(x19),I_(y19).

If one now wants to ensure that the electron beam hits the points P₉-P₁₉exactly, despite the existing disruption factors, one need only supplythe current amplitudes associated with these points and ascertained bythe training procedure to the corresponding x,y deflection units.Although direct currents are preferably employed in the training processas deflection currents, the correction factors obtained also apply tothe amplitudes of alternating currents.

By the association of the spatial x,y coordinates with the respectivecurrent coordinates, a transformation is performed in a certain sensefrom the space domain of the metal surface to the current domain of thedeflection coils. These current coordinates involve, as alreadymentioned, the actual current amplitudes that must flow in order for theelectron beam to approach a prescribed x,y point. It would also bepossible, however, to assign to each x,y point an ideal currentcoordinate and to the latter in turn a correction factor, which could beexpressed by I_(ideal)/I_(actual).

The result of the training process for a rectangular crucible 60 isillustrated in FIG. 4a and 4 b. Here one can recognize a crucible 60with the points P₁-P₂₂ which are to be approached exactly. In order toachieve this exact approaching by means of an electron beam that issubject to the aforementioned static deflection error, the currents forthe x and y coordinates must have the values that are shown in 61. Thesecurrents labeled by the points P₁′-P₂₂′ are corrected and do not lie ona rectangle, but rather on a distorted trapezoid.

The deflection coils must, as designated by 61, be subjected to thecorrected current amplitudes in order for the electron beam ultimatelyto strike the points P₁ . . . P₂₂ of the crucible.

The point grid shown for the crucible 60, which is sufficient for theteach-in process, is nonetheless relatively coarse for the operation. Inorder to be able to apply the correct current to the deflection coilseven for the intermediate areas, interpolation methods can be employed.A particularly suitable method for obtaining a greater point densitythan shown at 60 in FIG. 4 exists in the method of least squares. Inthis method, a compensation polynomial$y = {\sum\limits_{i = 0}^{n}{a_{i}x^{i}}}$

is determined for n measured value pairs x_(i),y_(i). Serving as acompensation criterion is the sum of the squared deviations of theindividual measurement points from the compensation polynomial, whichshould be minimal (H. Frohner, E. Ueckert: Grundlagen der elektrischenMeβtechnik (Foundations of electrical metrology), 1984, pp. 208, 209).In this compensation process the respective currents, that is to say,also the currents for the intermediate positions between the points P₁ .. . P₂₂, are ascertained via the equations $\begin{matrix}{{{current}\quad {amplitude}_{k}} = \quad {\sum\limits_{i,{j = 0}}^{n,m}{a_{i,j,k} \cdot ( {{spatial}\quad {coordinate}\quad x} )^{i} \cdot}}} \\{\quad ( {{spatial}\quad {coordinate}\quad y} )^{j}}\end{matrix}$

(spatial coordinate x)^(i)·(spatial coordinate y)^(j)

wherein

k=x′,y′ and

a_(i,j,k)=the coefficients that were determined in the course of thetraining procedure at the points P₁ . . . P₂₂.

In order to correct the aforementioned errors for all coordinates, thedeflection errors are thus calculated by way of a two-dimensional nthorder polynomial, that is to say, the current amplitude (I_(x),I_(y))can be calculated by way of this polynomial for any arbitrary point(x,y) in a plane.

By this process, even the places not measured during the teach-inprocess are transformed from the space domain into the current domain.

After the static deflection correction has been performed, a dynamicfrequency correction is undertaken, i.e., a corrected alternatingcurrent is determined from a purely sinusoidal alternating currentapplied to the deflection coils. This alternating current correction isnecessary because frequency errors occur in practical operation. Whatthe frequency errors involve is explained below on the basis of FIG. 5.

In FIG. 5, which shows a one-dimensional motion of an electron beam, apurely sinusoidal deflection of the electron beam 7′ is illustrated. Inthis case, the deflection of the electron beam 7′ is determined by theamplitude and the frequency of the current I=I₀·sin ωt flowing throughthe deflection unit, which results because of the voltage U=U₀·sin ωtsupplied by the power supply 12. For a predetermined current intensityI₀ and a predetermined circular alternating-current frequency ω₁, theelectron beam 7′ moves from a first position I to a second position IIand back again, specifically in the cycle frequency of the respectivealternating-current frequency. The electron beam 7′ thus describes astraight line drawn between I and II on the surface. The deflectionangle α₁ to the right of the central axis 16 corresponds in this case tothe deflection angle α₁′ to the left of the central axis 16. Both anglesα₁,α₁′ are determined by the current intensity of the respectivealternating current flowing through the deflection unit 11. If thealternating-current frequency is increased with a constant currentintensity, then theoretically the electron beam 7′ will continue tooscillate between the positions I and II, but at a higher frequency.

In practice, however, the deflection angles α₁ and α₁′, respectively,are dependent on the frequency because a so-called frequency attenuationoccurs. The frequency-dependent attenuation comes about essentiallybecause of eddy-current losses in the magnetic deflection system. Sincea deflection system does not consist only of a coil, but also containsiron, frequency-dependent reactions on the coil current result from theeddy currents flowing in the iron.

Additionally, a nonlinear frequency characteristic of the amplifiersupplying the coils can, in particular, lead to a frequency dependenceof the coil current. Ultimately, this means that for an equal currentintensity but an increased frequency ω₂, the deflection angles α₂ andα₂′ are smaller than α₁ and α₁′. The difference α₁−α₂ between these twoangles is also called the phase shift θ of the frequency attenuation.This frequency attenuation θ is a function of the alternating-currentfrequency, i.e., θ=f(ω).

A phase shift θ can also occur when the deflection systems for the x andy directions are different. This phase shift θ signifies a time-domainoffset of the currents in the respective deflection coils responsiblefor the x and y deflection directions.

In the upper representation 31 in FIG. 6a it is shown how the currentamplitude of a deflection unit in the x direction must change withincreasing frequency in order to correctly approach a certain point withthe electron beam. The representation 32 in FIG. 6b shows, on the otherhand, how the amplitude in the y direction must increase with increasingfrequency so that a prescribed point can be correctly approached. Ineach case, the amplitude must be increased in order to compensate forthe attenuation factor which appears with increasing frequency. Theaforementioned functional connection amplitude=f(ω) can be determinedexperimentally for different frequencies and learned, that is to say,stored in a memory. For these purposes, by way of example, six differentfrequencies that must be detected during the training are sufficient. Incontrast to the training of static deflection errors, the electron beamneed not be directed to different points P₁ . . . P₁₉; rather, it issufficient, considering one coordinate, if it oscillates between twopoints, for instance, P₉ and P₁₀. For each of the six frequencies theamplitude is then changed sufficiently that the electron beam reachesthe points P₉ and P₁₀ .

In practice, this teach-in process is performed for two coordinates. Inthis regard, a sinusoidal current profile is specified for the twodeflection units with the frequency being varied. If a frequencyattenuation occurs, the amplitude of an imaged ellipse or of an imagedcircle or a line decreases. The current amplitudes for the x and ydirections are now readjusted such that the prescribed deflectionamplitude is again achieved.

Due to the frequency attenuations, however, not only the amplitudes ofthe currents supplying the deflection units must be changed, but acorrection of the curve shape must also take place. If a purelysinusoidal current is fed to the deflection units from a voltage source,then a distortion of the sinusoidal form results because of dynamicerrors, i.e., the current deflecting the electron beam does not have apure sinusoidal form, but rather a sinusoidal form with superimposedharmonics. In order to be able to reproduce this distorted sinusoidalcurve, a Fourier transform is carried out. With the aid of the Fouriertransform process, a time-dependent function can be converted into afrequency-dependent Fourier transform and vice versa. The Fouriertransform thus represents an operation that generates a function in thefrequency domain from a function in the time domain, thus bringing abouta transition from the time to the frequency domain and vice versa. If acurrent running over time that is sent through the deflection units isFourier-transformed, then several frequencies with different amplitudesresult.

In the upper part 57 in FIG. 6c, the amplitudes of the Fouriercoefficients for the y component of the deflection current of anelectron beam are presented, while in the lower part 58 in FIG. 6d, theFourier coefficients for the x component of the deflection current arepresented.

For a transformation from the time to the frequency domain, the use ofthe Fast Fourier Transform (FFT) is always to be recommended if adigital computer is being employed.

The Fast Fourier Transform (FFT) is an algorithm for the calculation ofFourier coefficients which requires considerably less calculation effortthan the conventional calculation methods (cf. J. W. Cooley and J. W.Tukey, “An algorithm for the machine calculation of complex Fourierseries,” Math. of Computers, Vol. 19, pp. 297-301, April, 1965).

The FFT involves a method for the efficient calculation of the DiscreteFourier Transform (DFT) of time sequences, that is to say, discrete datapatterns. The DFT is an independent transformation like the Fourierintegral transformation or the Fourier series transformation, forinstance.

What the Fourier transform means for continuous waves is what the DFTmeans for pulse samples taken according to the sampling theorem, knownas Nyquist samples. Thereby the DFT is particularly useful for powerspectrum analysis and for filter simulation by means of digitalcomputers.

No matter whether the normal or the Fast Fourier Transform is performed,it is of significance for the present invention that a correction takesplace in the frequency domain. This means that during the teach-in forcertain specified frequencies those (distorted) currents which generatethe desired scanning pattern on the surface of the material to bevaporized are subjected to a Fourier transform.

This is shown in FIG. 6c and 6 d for the frequency 1.631 kHz and,specifically, separately for the x and y components of the current. Thex component is presented in field 57 while the y component is presentedin field 58. If a purely sinusoidal frequency is supplied to thedeflection units, then it must be corrected in terms of current in orderto generate the desired deflection pattern. The correction is done herein that the sinusoidal profile of the frequency is Fourier-transformedand the Fourier coefficients are corrected by means of FFT correspondingto the values determined in the teach-in process.

The corrected current profile I′(ωt) is then obtained by inversetransformation. $\begin{matrix}{{I( {\omega_{T}t} )} = {\sum\limits_{l}{F_{l} \cdot ^{{l \cdot }\quad \omega_{T}t}}}} \\{{I^{\prime}( {\omega_{T}t} )} = {\sum\limits_{l}{F_{l}^{\prime} \cdot ^{{{l \cdot }\quad \omega_{T}t} - {\quad {\theta {({l \cdot \omega_{T}})}}}}}}} \\{F_{l}^{\prime} = {F_{l} \cdot \frac{\alpha ( {I,{\omega = 0}} )}{\alpha ( {I,{\omega = {l \cdot \omega_{T}}}} )}}}\end{matrix}$

wherein

I=ideal current of the deflection unit

F=Fourier amplitude

l=integer number

I′=corrected current

F′=corrected Fourier amplitude

α=deflection of the electron beam.

If one wishes to direct the electron beam deliberately by means of analternating current onto a point of the surface 15, it is thus notsufficient merely to adjust the current intensity such that acorresponding intensity of the direct current would achieve the desireddeflection; rather, the current intensity of the alternating currentmust be adjusted such that the phase or angle error is compensated.These considerations applying to purely alternating current also applyin principle to pulsed direct currents.

In addition to the frequency attenuation, i.e., the attenuation of thedeflection amplitude of the electron beam, a phase-angle rotation canalso occur as already mentioned above. If the deflection system is notsymmetrical in the x and y directions, for instance, because ofdiffering pole piece dimensions, a phase shift between these twodirection occurs. This phase shift or phase error is also correctedaccording to the invention, and specifically, in the following way.

For different frequencies, a sinusoidal current profile is again fed tothe deflection, so that a diagonally running line results on thecrucible. If a phase-angle rotation now occurs, an ellipse results fromthis line. A phase difference is then adjusted manually at differentfrequencies so that a line again becomes visible. When one has done thisfor several measurement points, six, for instance, one can againcalculate a necessary phase shift for each arbitrary frequency with apolynomial approximation.

During training or the teach-in, only perhaps six frequencies areFourier transformed. If one wishes to determine the corrected currentprofiles through the deflection coils for the other frequencies, then,as already indicated above, an approximation procedure must be utilized.This consists, for instance, in calculating the correction factor for anarbitrary frequency ω_(q) according to the following formula:$\frac{\alpha ( {I,{\omega = 0}} )}{\alpha ( {I,{\omega = \omega_{q}}} )} = {\sum\limits_{i = 0}^{n}{a_{i,l} \cdot \omega_{q}^{i}}}$

wherein l=x or y

a_(i,l)=coefficients that were determined by the teach-in procedure.

In FIG. 7, a schematic representation of the conduct of the inventionfor an x deflection is shown. The voltage supplied by a power supply 12is labeled U_(d)=f(x) here, which is intended to express the fact thatthe current can have an arbitrary form, i.e., that it can be a direct oran alternating current, and is provided for the supply of thex-deflection coil.

This current I_(d)=f(t) from the current source 12 is subjected by meansof an appropriate unit in block 40 to the static deflection errorcorrection described above. The result ensuing from this is subjectedaccording to the above-explained process to a dynamicfrequency-attenuation correction in a unit 41. The current corrected inthis manner has the effect that the electron beam 7 strikes exactly thex coordinate that it is supposed to strike.

As results from the above, geometric x,y data designating the point tobe covered by the electron beam is first specified for the invention.This may, for instance, be a circular or ellipsoidal surface. Then thesecoordinates are associated with corrected current values which correctthe deflection errors. In place of the input of spatial coordinates intoa computer it is also possible to input directly the power distributionthat an electron beam should generate on a surface. The current profilethus obtained is subsequently transformed for the time domain into thefrequency domain and corrected in this frequency domain by correctingthe amplitudes of the individual harmonics, that is to say, theamplitudes of the higher frequency components are increased relativelymore strongly than the amplitudes of the lower frequency components.Thereupon, the corrected frequency spectrum is transformed back into thetime domain, so that a corrected current profile results.

Static and dynamic error correction represents a precondition for thisdirect input of power distributions. If an error correction was notundertaken, the specified power distribution cannot be reproducedeither.

The specification and performance of the power distribution will bedealt with further below.

A rectangular crucible 60 is shown in FIG. 8, in which the deeplydarkened fields 61,62 represent a large electron-beam impact power,while the less strongly darkened fields 63,64,65 represent a smallerelectron-beam impact power.

The crucible 60 has, for instance, a length of one meter and width ofapproximately 10 cm. The crucible 60 is shown together with the gridfields 66,67 on a computer monitor screen 68. The desired powerdistribution 68 in the x direction and the desired power distribution 69in the y direction can be input with the aid of a light pen or the like.

According to the invention, this power distribution is producedautomatically. This happens in that the path of the electron beam iscalculated automatically and taking into account the aforementionederror corrections. The result of this calculation is shown in FIG. 9,where one recognizes the path 70 of the electron beam. The powerdistribution on the rectangular crucible 60 is thus specified by way ofa longitudinal and a transverse profile P_(x)(x),P_(y)(y). The powerdistribution on this surface then results from the multiplication

P({overscore (r)})=P_(x)(x)·P_(y)(y); {overscore (r)}={x,y}

If, rather than a rectangular crucible, a round one is involved, thespecification can be done similarly.

In FIG. 10, a round crucible 75 with a certain nominal powerdistribution is shown. This crucible 75 is represented on a monitorscreen with polar coordinates 77 and a radius-power plane 78 on amonitor screen 79. The curves 80,81 can now be specified by means of alight pen or other suitable measures, whereby the power distribution ofthe crucible is also established.

The path 82 of the electron beam which is associated with the powerdistribution shown in FIG. 10 is represented in FIG. 11. In the case ofthe round crucible 75, a radial and an azimuthal power distributionP_(r)(r) ,P_(φ)(φ) are specified. The resultant power then is obtainedfrom

P({overscore (r)})=P_(r)(r)·P_(Φ)(Φ)

wherein {overscore (r)}={r·cos(Φ), r·sin(Φ)}

The frequencies ω_(x),ω_(y) or ω_(r),ω_(θ) of the deflection currentsare in principle freely selectable. The resolution and shape of thedeflection pattern can be influenced by the suitable choice of thefrequency difference. This starting and ending points of the beam pathare defined by way of the phases ρ_(x),ρ_(y) or ρ_(r),ρ_(θ). Averagedover the time T=1/ω, the above defined power profile results,independently of the selection of frequencies and phases.

In particular, the frequency difference can be selected to be small,while simultaneously a deflection pattern can be generated that shows ahigh resolution. Thus the deflection frequencies are similar in thehorizontal and the vertical direction. This reduces the requirement onthe deflection system with regard to transmission characteristics ofhigh frequencies (counter example in television technology: the linefrequency (horizontal deflection) is considerably greater here than thepicture frequency (vertical deflection)).

In FIG. 12, a schematic block diagram for the driving of twoelectron-beam guns 90,91 by means of a computer 92 is shown. Thiscomputer 92 can communicate with a control unit 93. The electron-beamguns 90,91 each generate an electron beam 94,95, whose position andintensity are specified by the computer 92. Signals are issued by thiscomputer 92 to transputers with D/A converters 96,97, which in turn eachdrive an amplifier 98,99, each of which is connected to one of the twoelectron-beam guns 90,91. A transputer is understood to mean a 32-bitchip and computer which was developed by the British firm INMOS andserves above all for the parallel, i.e., simultaneous, processing ofextensive amounts of data. An ordinary microprocessor is accompaniedhere by four communication units, which can each transfer 10 MB of dataeven during the computational activity of the processor. Thuscomputation-intensive tasks can be managed in a very short time.

FIG. 13 shows a closed control loop for a device according to theinvention. Here 101 labels a crucible, whose vaporization products endup on a rolled film 102. The measurement data picked up from thecrucible 101 or the film 102 by means of measuring unit 103,104,electrical resistance values, for instance, or optical transmissivities,are supplied to a PID regulator 105, whose control signals arerecalculated in a device 106 into electron-beam deflection patterns andsupplied to the transputers 96,97 which drive the electron-beam guns90,91.

The essential steps in the implementation of the invention are indicatedin FIG. 14. It is possible to specify either a power distribution or ageometrical pattern. If in block 112 a geometrical pattern, a circle,ellipse, etc., for an electron beam is specified, then the spatialcoordinates are transformed at 113 into deflection currents with staticcorrection. These statically corrected deflection currents are subjectedat 114 to a dynamic frequency correction. The inverse-transformed resultis supplied to the transputer at 115.

At the left side of FIG. 14 it is shown that at 110 a power distributionis specified in place of a geometrical pattern. The geometrical form iscalculated at 111. The further sequence is then the same as for thespecified geometrical pattern.

Further variations and modifications of the foregoing will be apparentto those skilled in the art and are intended to be encompassed by theclaims.

German priority application 197 45 771.1 is relied on and incorporatedherein by reference.

We claim:
 1. A method for the operation of an electron beam which isemployed for the vaporization or melting of a material in a crucible,comprising: providing at least one deflection unit for the electronbeam, said at least one deflection unit providing image defects;selecting geometrical coordinates (x, y; r, φ) of at least one point onthe surface of the material to be vaporized or melted; using anerror-correction algorithm to transform said selected geometricalcoordinates (x, y; r, φ) into currents (I_(x), I_(y); I_(r), I_(φ)),wherein said error-correction algorithm is used for correcting the imagedefects of said at least one deflection unit; supplying the currents tosaid at least one deflection unit; directing with said at least onedeflection unit the electron beam at an intensity onto the material tobe vaporized or melted; and guiding the electron beam at a specifiablevelocity over said at least one point of the surface of the material tobe vaporized or melted.
 2. The method according to claim 1 wherein thecurrents are statically corrected.
 3. The method according to claim 1wherein the currents are dynamically corrected.
 4. The method accordingto claim 1 wherein the currents are first statically and thendynamically corrected.
 5. The method according to claim 1, wherein theintensity of the electron beam is changed.
 6. The method according toclaim 1, wherein the intensity of the electron beam is essentiallyconstant.
 7. A method for the operation of an electron beam which isemployed for the vaporization or melting of a material in a crucible,comprising: providing at least one deflection unit for the electronbeam, said at least one deflection unit providing image defects;establishing a geometrical power distribution of the electron beam onthe surface of the material to be vaporized or melted; determininggeometrical coordinates (x, y; r, φ) of the power distribution on thesurface of the material to be vaporized or melted; using anerror-correction algorithm to transform said geometrical coordinates (x,y; r, φ) into currents (I_(x), I_(y); I_(r), I_(φ)), wherein saiderror-correction algorithm is used for correcting the image defects ofsaid at least one deflection unit; supplying the currents to said atleast one deflection unit; directing with said at least one deflectionunit the electron beam at an intensity onto the material to be vaporizedor melted; and guiding the electron beam at a specifiable velocity overat least one point of the surface of the material to be vaporized ormelted.